This proposal aims to understand the impact of a pathogenic lentivirus infection on the immune system in aging, a physiologic state of immune impairment. This question is significant in that it addresses a critical public health issue, given that numbers of people with HIV over age 50 is on the rise due to both, dramatic efficacy of potent combination antiretroviral therapy (cART) and increasing incidence of new HIV infections in the aging population, It is well established that robustness of vaccine -induced immune responses are a powerful tool for elucidating level of immune competence and a useful model for investigating mechanisms of immune dysfunction. Our studies with peripheral blood from HIV infected people on cART have indicated that Ab responses to seasonal influenza vaccine are impaired with aging and are associated with increased immune activation and defective function of a CD4 subset designated as peripheral T follicular helper cells (pTfh) jn blood. A subset of these cells are believed to be functionally similar to germinal center (GC) Tfh that provide critical help to B cells for differentiating into antibody producing cells, but the relatioship of pTfh to GC Tfh and Tfh traffic from blood to lymphoid tissue has not been conclusively established. Based on our findings we hypothesize that 1. Aging is associated with functional deficiency of Tfh that leads to impaired vaccine responses and 2. These defects are made worse by HIV infection despite virologic control with cART and 3. Immune activation exacerbates Tfh dysfunction. The Tfh-B cell interaction can only be answered through study of lymphoid tissue which is most feasible in rhesus macaques. Our team has special expertise in in situ analyses of lymph nodes that will enable us to investigate Tfh and B cell interaction in situ. . We will compare young and old SIV infected macaques with controlled viremia for their responses to seasonal influenza and tetanus vaccines for T-dependent immune responses to study neo and recall antibody responses. SIV uninfected RM will serve as controls. We will investigate the mechanisms of impaired vaccine responses by characterization of total and antigen-specific Tfh and B cells by phenotype, cellular function and gene transcription studies in cells from lymph nodes and peripheral blood. In situ immunohistology and histo- cytometry studies will provide novel insights into the impact of aging and that of aging complicated by SIV infection on Tfh and B cells in GC. We will study if cells with Tfh phenotype traffic to LN in association with booster vaccination. We will define more precisely the role of underlying inflammation and immune activation in perpetuating immune dysfunction and identify predictive immune signatures of vaccine responsiveness. Subcutaneous injections of interleukin 21 (the signature Tfh cytokine) will be investigated as a modality to augment vaccine responses. These findings have relevance for HIV infected aging population.